The present invention relates generally to the fabrication of uranium-alloy articles into desired configurations, and more particularly to fabricating articles of uranium alloys exhibiting shape memory effects wherein the method of the present invention circumvents the memory behavior of the alloys.
Uranium is often alloyed with various metals to improve the resistance of the uranium to corrosion. Alloys of uranium containing niobium (5-23 wt.%) and zirconium (0-10 wt.%) afford satisfactory corrosion resistance. To obtain the desired corrosion resistance, these alloys are usually heated to a temperature greater than the gamma phase formation temperature (about 647.degree. C.) and then rapidly quenched to ambient temperature to transform the gamma phase by a martensitic process to a transition phase which is a monoclinic distortion of the orthorhombic alpha-uranium structure that is metastable at ambient temperature. It was previously discovered that such uranium alloys when stressed or deformed while in this martensitic state to a selected configuration recover their preformed shape when heated to temperatures greater than about 100.degree. C. This shape recovery of the uranium alloys is known in the art as shape memory effect. It was found that up to a certain deformation strain about 100% of the original shape was recoverable by the heating of the formed articles. This pseudo-plastic behavior of the uranium-niobium alloys is described in U.S. Pat. Nos. 3,567,523 and 3,802,930 dated Mar. 2, 1971 and Apr. 9, 1974, respectively. These patents relate to processes in which the pseudo-plastic behavior of uranium-niobium alloy is utilized in selected configurations to demonstrate complete reversability of the deformed configuration at both below and above the deformation temperature. Further discussion of this shape memory effect of uranium alloys is set forth in the publication Metallurgica, Vol. 12, pp. 243-248 (1978), printed by Pergamon Press, Inc., and entitled "Shape Memory Effects in a Uranium+14 at .% Niobium Alloy," by R. A. Vandermeer et al. In the aforementioned patents and publication the shape memory effects of uranium-niobium alloys are described in detail and stress important advantages achieved by utilizing such memory effects in area such as bimetallic strips and other heat-controlled configurations.
However, while the shape memory effects of these uranium alloys are often desirable, there are many instances where the memory effect of the alloys is not desirable and in fact is detrimental to the intended use of the alloy. For example, the shape memory effects would be undesirable in structural applications of the alloy.
In applications where the memory effect is not desirable it is common practice to form parts above the 647.degree. C. transformation and then to water quench to the metastable phase. The distortion resulting from the quench and the roughened, hot-worked surfaces are generally eliminated by subsequent machining. However, for thin gage, non-symmetrical components this manufacturing sequence is generally uneconomical and in many cases practically impossible. Thin-gage, non-symmetrical parts are generally formed by established sheet metal working techniques at ambient temperatures. However, for the said uranium-niobium and uranium-niobium-zirconium alloys these established sheet forming techniques yield components which are extremely dimensionally unstable during thermal cycling. For example, if a uranium-6 wt.% niobium flat plate is formed into a configuration with 5 to 10% plastic strain by established sheet working techniques at 23.degree. C., and the part is subsequently heated to 150.degree. C., the part will revert to its orignial flat shape during the thermal cycle.
To obtain the desired dimensional stability and corrosion resistance, the part must be quenched from a temperature at which it is in the gamma phase after it is fully formed to the desired shape. When a cold formed part is heat treated it loses its formed shape due to shape memory in the upward heating cycle.
The problem cannot be solved by intermediate temperature forming due to the transformation to phases with undesirable corrosion resistance (alpha uranium) at temperatures between the gamma transformation temperature and the shape change temperature.